There are a variety of switching power converters capable of four-quadrant output (where the output voltage can be positive, negative or zero, and the output current can also be positive, negative or zero). The majority are derived from a Buck topology. Buck converters are limited in that their maximum output voltage must be less than their input source voltage. In the case of two buck converters operated in a bridge, the maximum output voltage must be less than twice the input source voltage. In applications where the available input source voltage is fixed, an intermediate power conversion may be necessary to step up the available input voltage to achieve the required output voltage. This increases cost and complexity.
Four-quadrant converters have been made using a topology other than Buck to avoid this output-voltage limitation. U.S. Pat. No. 4,186,437 by Slobodan M. Cuk describes a novel four quadrant converter and a boost-derived four-quadrant converter. Both of these topologies have no theoretical limitation on their maximum output voltage. In both topologies, four-quadrant operation is achieved by operating two converters in phase-opposition and taking the output differentially across the outputs of the two converters in a bridge configuration. Due to the use of two converters in a bridge configuration and the resultant differential output, neither output terminal may be common with a terminal of the input voltage source. This lack of a common terminal prevents the desirable capability of combining the outputs of two such four-quadrant converters to a single load in a bridge configuration. Further, two or more inductors are required in either topology.
There are several converters that have some of the same elements of the present invention, but fail to function in a manner that would provide a four quadrant output and/or an unconstrained output signal. For example, U.S. Pat. No. 6,222,352 to Ronald J. Lenk (“Lenk”) has three operational modes, each of which occurs for a controlled time. Lenk does this to independently control two different outputs. Each of the outputs in Lenk operates a single voltage-current output (positive voltage, positive current) as opposed to all four quadrants. Furthermore, each output in Lenk is constrained to be smaller than the input.
U.S. Pat. No. 6,429,629 to Tranh To Nguyen (“Nguyen”) discusses converters that have a center-tapped wound magnetic element or transformer with a DC input applied to the center tap and switches from each winding end to ground. The converters in Nguyen only operate in one voltage-current quadrant (positive voltage, positive current or negative voltage, negative current) as opposed to all four quadrants. Once again, the output voltage magnitude is constrained by the input voltage. Nguyen states that the converters shown in FIGS. 3A–3J are capable of output of “any polarity and magnitude.” However, it must be understood that all of the converters in Nguyen operate inherently in one quadrant depending on the direction of the diodes or synchronous rectifiers and that the output magnitude is constrained by the input voltage and turns ratio of the transformer.